The present invention relates to a liquid crystal display based on an active matrix system, and more particularly to a liquid crystal display in which MOS transistors on a single crystal silicon substrate or Thin-Film Transistors employing poly crystal silicon are used.
In order to clarify the present invention, a conventional active matrix driving system will be explained below: Incidentally, concerning an active matrix panel technology known up to the present time, it is explained in detail in Shunsuke Kobayashi. xe2x80x9cColor Liquid Crystal Displayxe2x80x9d (published from Industrial Library Ltd. in 1990). Moreover, concerning a technology for preventing a flicker caused by leakage resistance of liquid crystal, it is described in JP-A-6-118912.
A liquid crystal display based on the active matrix system, in which MOS (Metal-Oxide Semiconductor) transistors on a single crystal silicon substrate or Thin-Film Transistors (TFT) employing poly crystal silicon are used, comprises a display unit and a driving circuit unit. The display unit is a unit in which transistors are located at the intersections of data signal lines and scanning signal lines arranged in a matrix-like structure. The driving circuit unit controls voltages for the data signal lines and the scanning signal lines.
In a transistor in the display unit, the gate is connected to a scanning signal line, the drain to a data signal line, and the source to a liquid crystal capacitor. Usually, a holding capacitor is added in parallel with the liquid crystal capacitor. Here, when the gate electrode comes into a selection state, the transistor is brought into conduction, thereby allowing an image signal on the data signal line to be written into the liquid crystal capacitor and the holding capacitor. When the gate electrode is changed into a non-selection state, the transistor has a high impedance, thus holding the image signal written in the liquid crystal capacitor.
The driving circuit unit comprises a scanning circuit for controlling the voltages for the scanning signal lines and a signaling circuit for controlling the voltages for the data signal lines. The scanning circuit applies a scanning pulse to each of the scanning signal lines once every one frame time. Usually, a timing of the scanning pulse toward each of the scanning signal lines is shifted in sequence from an upper side of a panel to a lower side thereof. A time of 1/60 second is often employed as the one frame time. In a panel of 640xc3x97480 dots, i.e. a representative pixel configuration, since 480 scannings are performed during the one frame time, a time width for the scanning pulse becomes equal to about 35 xcexcs. A shift register is commonly used in the scanning circuit, and an operating rate of the shift register is equal to about 28 kHz.
Meanwhile, the signaling circuit applies, to each of the data signal lines, a liquid crystal driving voltage the value of which is equivalent to driving liquid crystal of pixels by a single row to which the scanning pulse is applied. In a pixel to which the scanning pulse is applied, a voltage of a gate electrode of the transistor, which is connected to a scanning signal line, becomes high, and thus the transistor is switched to ON state. At this time, the liquid crystal driving voltage is applied to the liquid crystal from a data signal line by way of a drain and a source of the transistor, thus charging a pixel capacitor comprising the liquid crystal capacitor and the holding capacitor. Repetition of this operation allows a signal voltage corresponding to an image, every frame time and repeatedly, to be applied to a pixel capacitor over the entire surface of the panel.
The liquid crystal driving voltage applied to the liquid crystal, by inverting the polarity thereof every frame time, is converted into an alternating voltage. When a frame frequency is equal to, as usual, 60 Hz, a liquid crystal driving frequency becomes equal to 30 Hz, i.e. one-half of the frame frequency. Also, the liquid crystal-driving voltage converted into the alternating voltage with positive and negative polarities is distorted by crosstalk, which is caused by the gate voltage when the transistor is switched from ON state to OFF state, or by the leakage resistance of the liquid crystal.
At the liquid crystal driving frequency of 30 Hz, the distortion of the liquid crystal-driving voltage causes people to feel and see a flickering light called flicker. In order to prevent the flicker from being perceived, it can be considered that a period of the liquid crystal driving voltage (a specific period of the voltage applied to pixel electrodes and having different polarities) is made shorter so that the flicker becomes imperceptible to human eyes. However, it is difficult to fabricate, with a stable yield, an active element for driving pixel electrodes in the conventional liquid crystal display. Also, as a method of making the flicker difficult to recognize visually with human eyes, there exists a driving method in which polarities of driving voltages applied to adjacent pixels are inverted. This is a method of applying signal voltages, the polarities of which are obtained by mutually inverting polarities of signal electrodes of the pixels which are adjacent in a right-to-left direction and those of signal electrodes of the pixels which are adjacent in an up-and-down direction.
It is an object of the present invention to provide a liquid crystal display which produces no flicker.
It is another object of the present invention to provide a liquid crystal display which, by miniaturizing transistors therein and lowering a withstanding voltage thereof, has a large aperture ratio and consumes less electric power.
In a liquid crystal display based on the conventional active matrix driving system, there occur the following problems:
First of all, a serious problem is the occurrence of the flicker. In the above-described liquid crystal display based on the active matrix driving system, the liquid crystal-driving voltage, by inverting the polarity thereof every frame time, is converted into the alternating voltage. As a result, when the frame frequency is equal to, as usual, 60 Hz, the liquid crystal driving frequency becomes equal to 30 Hz, i.e. one-half of the frame frequency. At the liquid crystal driving frequency of 30 Hz, the flickering light called flicker becomes perceptible. In order to prevent the flicker from being perceived, the driving method, in which polarities of driving voltages applied to adjacent pixels are inverted, is employed. This is the method of applying signal voltages, the polarities of which are obtained by mutually inverting polarities of signal electrodes of the pixels which are adjacent in a right-to-left direction and those of signal electrodes of the pixels which are adjacent in an up-and-down direction. The polarities of the signal electrodes, in the case of the above-mentioned panel of 640xc3x97480 dots, are inverted every one scanning time period, i.e. 35 xcexcs. Accordingly, a driving frequency for the signal electrodes becomes equal to 14.4 kHz, i.e. about 500 times as great as the liquid crystal driving frequency. This situation brings about a decrease in the design flexibility.
Also, in the above-described driving method, when displaying a specific pattern such as a checkered pattern obtained by simultaneously displaying pixels to which voltages with an identical polarity are applied, the flicker becomes so conspicuous as to be recognized visually with human eyes.
A second problem is a high withstanding voltage of the transistors. In the above-described liquid crystal display based on the active matrix driving system, sampling of the voltage, the polarities of which are inverted every frame time by the transistors in the display unit, is performed, thereby controlling the liquid crystal-driving voltage. This requires that a withstanding voltage of the transistors in the display unit should be two times or more of an effective voltage for driving the liquid crystal, thus resulting in so much consumption of electric power. Meanwhile, concerning a small-sized and high resolution liquid crystal panel for a liquid crystal projector or a super high resolution liquid crystal display, in order to enhance the aperture ratio, the transistors are expected to be miniaturized. When miniaturizing the transistors through the microprocessings, the withstanding voltage thereof becomes an extremely serious obstacle.
A third problem is a reduction in impedance for driving the liquid crystal. In the above-described liquid crystal display based on the active matrix driving system, a transistor in the display unit allows an image signal, the sampling of which is performed using a scanning signal on a scanning signal line, to be applied to a holding capacitor and a liquid crystal capacitor, thereby controlling the liquid crystal. This makes it impossible for impedance of the liquid crystal to continue applying the voltage over one frame time period, i.e. a sampling period. In order to hold the voltage, a sufficiently large capacitor is needed. The impedance becomes a serious obstacle especially when guest host type liquid crystal is applied to the liquid crystal display.
Although, as described above, the present invention makes it possible to solve many problems, an important object thereof is to provide a liquid crystal display which allows the flicker to be eliminated.
A constitution of the present invention can be considered as follows: In a liquid crystal display which has a pair of substrates, a liquid crystal layer held by the pair of substrates in such a manner as to be sandwiched therebetween, and, on one of the pair of substrates, a plurality of scanning signal lines and a plurality of data signal lines which are formed in a matrix-like structure with reference to the plurality of scanning signal lines, a plurality of pixels are constituted in domains surrounded by the scanning signal lines and the data signal lines, and a pixel circuit, which applies to the liquid crystal layer, is formed in each of the pixels.
In this way, a pixel circuit formed in each pixel applies, to the liquid crystal layer, thereby making it possible to suppress the occurrence of the flicker.
Also, the pixel circuit is formed so that it has a first storing means for storing a liquid crystaldriving voltage for the present frame, a second storing means for storing a liquid crystal driving voltage for a one-preceding frame, and a switching means for switching between the first storing means and the second storing means.
Moreover, assuming that the liquid crystal driving voltage having a different polarity is a voltage obtained by alternately applying the liquid crystal-applied voltage for the present frame and the liquid crystal-driving voltage for a one-preceding frame, it becomes possible to apply, to the liquid crystal layer and without exerting essential influences on the other interconnections or without installing new interconnections, the one period or more of liquid crystal driving voltage which has a different polarity for every one frame.
As another means in the present invention, the following constitution can be considered: In a liquid crystal display which has a pair of substrates and a liquid crystal layer held by the pair of substrates in such a manner as to be sandwiched therebetween, one substrate of the pair of substrates has a plurality of first scanning signal lines, a plurality of second scanning signal lines formed between the plurality of first scanning signal lines, and a plurality of data signal lines formed in a matrix-like structure with reference to the plurality of first scanning signal lines and the plurality of second scanning signal lines, and a plurality of pixel circuits, which drive liquid crystal molecules connected to the first scanning signal lines, the second scanning signal lines and the data signal lines, are formed in domains surrounded by these interconnections, and each of the pixel circuits is constituted so that it has a first voltage holding means connected to a corresponding first scanning signal line and a corresponding data signal line so as to hold an image signal voltage from the data signal line, a second voltage holding means connected to a corresponding second scanning signal line and a corresponding data signal line so as to hold an image signal voltage from the data signal line, a switching means which, with the use of a switching signal, switches between an output voltage of the first voltage holding means and that of the second voltage holding means so as to output a switched output voltage, and a pixel electrode connected to the switching means so as to apply the output voltage from the switching means to the liquid crystal.
This constitution also allows the flicker to be eliminated substantially.
In this constitution, it is preferable that the first voltage holding means should hold an image signal voltage with a positive polarity and the second voltage holding means should hold an image signal voltage with a negative polarity.
Furthermore, operation periods of the first voltage holding means and the second voltage holding means are made different from an operation period of the switching control means. This allows the flicker to be eliminated even further.
Also, it is desirable that the switching means should switch between the first voltage holding means and the second voltage holding means at least one time or more within one frame time period.
Also, a common electrode is formed on the other substrate of the pair of substrates, and a voltage, the polarity of which is opposite to that of the liquid crystal driving voltage applied to a pixel electrode in a pixel circuit, is caused to be applied to the common electrode. This makes it possible to lower the voltage applied to the pixel electrode in the pixel circuit, thus enabling the power consumption to be lowered.
The first voltage holding means constituted as above is further provided with a first switching element, a first capacitance element and a first buffer amplifier. The second voltage holding means constituted as above is further provided with a second switching element, a second capacitance element and a second buffer amplifier. In addition, the first switching element is constituted by a first P type transistor, and the second switching element is constituted by a first N type transistor. This makes it possible to employ transistors having a low withstanding voltage, thus enabling the power consumption to be lowered.
Also, it is desirable that the first buffer amplifier should be a voltage follower circuit constituted by a second N type transistor, and the second buffer amplifier should be a voltage follower circuit constituted by a second P type transistor.
If the transistors formed in the pixel circuit constituted as above are all N type transistors or P type transistors, it becomes possible to employ transistors having a low withstanding voltage. This enables the power consumption to be lowered.
In these constitutions of the liquid crystal display according to the present invention, first and second voltage holding circuits hold an image signal with a positive polarity and an image signal with a negative polarity, respectively. A switching circuit switches between the outputs from the voltage holding circuits, and the liquid crystal is driven using the switched output signal. This makes it unnecessary to cause a timing, with which the image signals are written into the first and the second voltage holding circuits, to coincide with a timing with which the liquid crystal is driven. As a result, in the liquid crystal display according to the present invention, by shortening a period of a control signal for the switching circuit, it is allowable to increase a frequency at which the liquid crystal is driven. This makes it possible to prevent the flicker. Also, it is possible to convert a drain signal into an alternating signal having two frame periods, thereby allowing the power consumption to be lowered. Moreover, by lengthening, as required, a period for writing the voltage into the voltage holding circuits, it is also possible to lower the power consumption.
Also, in the liquid crystal display according to the present invention, it is possible to drive the liquid crystal with the use of a voltage obtained by adding an alternating amplitude applied to the common electrode to the image signals held by the first and the second voltage holding circuits. On account of this, it is sufficient for the pixel circuit to generate an amplitude which has a positive or a negative polarity and is varied in correspondence with an image signal. This makes it possible to embody the pixel circuit with the use of transistors having a low withstanding voltage. For example, in the case of a 5V-driven liquid crystal, the liquid crystal driving voltage falls in a range of 2V to 5V in terms of the effective value, and thus a withstanding voltage of transistors used was required to be 10V or more even in an ideal case. In the liquid crystal display according to the present invention, however, it is possible to employ a method in which 2V, i.e. a minimum voltage for driving the liquid crystal, is applied from the common electrode and 3V, i.e. a variation amount of the voltage with a positive or a negative polarity, is controlled by the first and the second voltage holding circuits and the switching circuit. Accordingly, it is sufficient that, ideally, the withstanding voltage of the transistors used in the pixel circuit is equal to 3V or more. This makes it possible to lower a withstanding voltage which the transistors are required to have, thus eventually making it possible to lower the power consumption of the whole liquid crystal display.